At present, many thin-walled rotational parts are of a structure with an integrally machined inner reinforcement rib, in order to reduce the weight, improve the effective load, and increase the strength thereof. Spinning process is widely used for forming parts with inner ribs, but it has a serious difficulty in forming ribs with a large aspect ratio. Especially for magnesium alloy materials with low plastic formability, ribs formed therefrom usually have a quality that cannot meet the requirements as they are prone to defects such as cracks, ripples, accumulation and fracture. The spinning process used currently does not well adapt to the magnesium alloys.
A fine-grained structure may be obtained through plastic deformation, which can also improve the strength and toughness of parts, thereby producing a good fine-grain strengthening effect. However, the traditional metal plastic processing methods, such as forging, extrusion and torsion, each produce a small plastic strain which is generally less than 2.5. If the number of forming passes is increased so that the corresponding plastic strain reaches 2.5 or more, the thickness and diameter of the processed material will become very small, which makes the material not suitable for forming structural parts in most cases. Studies show that micron and submicron grains can be obtained only under a true strain reaching at least 4.0 or more, so as to exhibit an excellent fine-grain strengthening effect. At present, some new SPD plastic processing technologies, such as ECAP (Equal Channel Angular extrusion), TE (torsion extrusion), HPT (high-pressure torsion) and CEC (cyclic extrusion compression), each enable the materials to obtain larger plastic strains, and they are thus effective ways to refine grains. However, these new technologies face severe challenges in the uniformity of forming. For example, in case of forming by high-pressure torsion, the strain at the center of a billet is much smaller than the strain at the edge portion of the billet, and the grain size of a microstructure at the center portion is also more than 2 times larger than that at the edge portion, that is, there is an extremely uneven deformation in the deformation section.